Hollow propeller blade with bulbed core



Sept 15, 1953 c. M. KEARNS, JR., 5TM. 29652912 HoLLow PROPELLER BLADEWITH BULBED come 'Film1 @une e, 195o L 3 sheets-shew 1 AGENT Sept 15,1953 c. M. KARNS, JR., ETAL 2,652,121

HOLLOW PROPELLER BLADE WITH BULBED CORE 5 Sheets-Sheet 2 Filed. June 6,1950 INVENTOFQS CHARLES M. KEAFZNS JR.

GLEN T. AMP'T'ON Y MFWM AGENT sept. 15, 1953 C. M. KEARNS, JR., ETM.

HOLLOW PROPELLER BLADE WITH BULBED CORE 3 Sheets-Sheet 3 Filed June 6,1950 INVENTORS CHARLES' IVI. KEARNS JR. GLEN T. I .AMPTON AGENT PatentedSept. 15,l i953 UNITED STATES PATENT OFFICE HOLLOW PROPELLER BLADE WITHBULBED CORE Charles M. Kearns, Jr., Wethersfield, Conn., and Glen T.Lampton, Culver City, Calif., assignors to United Aircraft Corporation,East Hartford, Conn., a corporation of Delaware Application June 6,1950, Serial N o. 166,458

2 Claims.

a hollow propeller blade having an improved core member upon which ismounted an outer blade shell.

A primary feature of this invention resides in the provision of a corehaving an increased area of attachment for the outer blade shell, theincreased area being located adjacent the inboard end of the shellthereby reducing stress concentrations due to bending in edgevvise andflatwise directions.

A. further object of this invention is to provide a hollow metal bladehaving a central core 1nember of the type described so as to reduce theamount of chordwise shell overhang beyond the core member.

A still further object of this invention is to provide a core memberhaving a relatively large bonded area of the core to the shell in theinboard section of the blade thereby providing a. greater area for loadtransfer from the shell to the core in the blade vicinity where stressconcentrations are known to be high.

Another object of this invention is to provide a hollow metal propellerblade having increased cross-sectional moments of inertia and sectionmoduli while having little or no increase in crosssectional areas of thecore and shell material in the inboard region of the blade.

These ano other objects will become readily apparent from the followingdetail description of the drawings in which:

Fig. l is a plan view of a tubular core member positioned in the lowerhalf of a bulbing die mechanism.

Fig., 2 is a slightly enlarged view similar to Fig. l illustrating thetubular core member after the bulbing operation has been performed.

Figs. and 4 are cross-sectional illustrations taken along the lines 3-3and Ll-i, respectively, of Fig. 2.

Fig. 5 is a cross-sectional View ofthe blade core member illustratingthe general configurations and wall dimensions of the core after anotherstep in fabrication thereof.

Fig. 6 is a cross-sectional view taken along the line 6-6 of Fig. 5.

Fig. 7 is a cross-sectional vievv of the core member after it has beenpartially flattened, the flat surface being illustrated in plan form.

Figs. 8, 9 and l0 are cross-sectional views taken along the lines S-,9-9 and itl-lil, respectively, of Fig. 7.

Fig. 1l is a plan View of the assembled propeller blade after the outershell has been positioned over the core member.

Figs. l2, 13, 4 and l5 are cross-sectional views taken along the linesI2-I2, |3-I3, llil and iii-l5, respectively, of Fig. 1l. Fig. l2 isenlarged for better illustration.

In hollow metal propeller blades of the type described herein, the coremember is the primary load carrying component, so it is desirable toimpart the necessary stiffness to the core and improve the load carryingcapacities thereof. To this extent, then, it is desirable to providehigh values of cross-sectional moments of inertia (section stiffness)about both major and minor section neutral axes in the region of theshank in order to achieve; first, high values of section modulus whichwill lower the vibration stress, and second, high natural vibrationfrequencies which will also tend to minimize vibration stresses.. Theseobjectives can best be achieved with a large diameter core whichprovides large values of section stiffness with minimum crosssectionalarea.

Referring to standard beam theory,

Bending moment B :H ending Stress Section modulus and Section modulus =IWhere I is the moment of inertia (section stiffness) and C isapproximately equal to one-half blade thickness. It can be seen that thestress can be minimized by increasing the section modulus, which can beincreased in turn by raising the moment of inertia I. This may be doneby adding to the Wall thickness of the hollow blade core.. Such aprocedure,l which makes little change in the core diameter, will resultin a relatively large increase in Weight for a small increase in Ibecause the added material is located near the neutral axes of thesection. The greatest increase for given weight is obtained byincreasing the diameter of the core since the mass is then moved agreater distance from the neutral axis and I increases with the squareof the distance between the mass and these axes.

The stress resulting from a vibratory excitation is a function of theamplitude of that vibration, and the amplitude increases rapidly as thefrequency of the excitation approaches the natural frequency of thevibrating system. Since the frequency of the exciting force acting upona propeller blade is determined by the rotational speed of thepropeller, it is therefore highly desirable to raise the naturalvibration frequency of the blade to a value well in excess of thefrequency represented by the rotational speed.

An increase in moment of inertia will increase the natural frequency ofthe blade since, for a uniform beam,

T N E I- l\atural frequency=\/U L4 where,

E :modulus of elasticity of the material U=mass per unit length I:cross-sectional moment of inertia Lzlength of the beam As stated above,an increase in the core moment of inertia can be achieved mostefficiently by increasing the core diameter.

The enlargement of the core member further provides a high ratio of coresection stiffness to shell section stiffness at the inboard end of theshell to minimize the stress concentration attendant upon the deliveryof the shell loads to the core. This feature can also be best achievedmost efficiently by a large diameter core.

At the same time enlargement of the core diameter provides a large areaof bond between the core and the shell at the shank to permit the use ofthe optimum configuration of the inboard end and also to minimize theamount of shell overhang at the leading and trailing edges in order toreduce the secondary shell stresses due to local deflections.

Utilizing a large diameter core over the full length of the blade,however, would result in excess weight and centrifugal twisting momentover the tip region of the blade where it is not required for structuralreasons. Such construction would possibly result in a blade tip sectionhaving greater thickness than that required for any other reason exceptto physically accommodate the core inside the leading and trailingedges.

A bulbed core as illustrated in this invention therefore obtains all theadvantages of a large diameter core at the shank while enabling the useof a more efficient, smaller diameter core at the tip thereby providinga blade with maximum strength and aerodynamic efficiency for a givenweight, centrifugal load and centrifugal twisting moment.

The particular construction of the blade and core of this invention isbest illustrated by describing the fabrication steps and method utilizedin producing a blade of this type.

Referring to Fig. l, an elongated metal tube 26 is shown having a wall22 of substantially uniform thickness and a blade inboard end 24 whichis slightly enlarged or thickened to provide for later machining so asto form elements of the blade retention mechanism. The core member 20may be positioned in a die 30 which has wall portions 32 and 34 whichsnugly contact the tubular core 20 and slightly enlarged walls 36 whichprovide the proper configuration for the core 2D when a portion thereofis expanded in diameter.

The die mechanism may include dowels 40 for aligning and positioning theupper encasing portion of the die assembly and may also include a rammechanism 44 which includes a plunger 46 for exerting a force at theblade tip end 48 of the core member 20 in an axial direction. A plug 5Useals off the outer end of the core member 20 while a plug 52 seals theroot end of the core member 20 to provide a fluid tight connection forhigh pressure hydraulic lines 56 and 58, respectively.

Extremely high uid pressure is directed to the central hollow portion 60of the core member 20 while a force is applied by the plunger 46 againstthe outer end of the core member so as to expand or bulge a portion ofthe core member to the dimensions of the wall 36 of the die. Inasmuch asthis bulging operation will somewhat shorten the length of the coremember 20, the plunger 46 of the ram mechanism 44 aids in overcoming thedie frictional forces along the walls 34. This bulging operation may beperformed in a single pass or in several individual passes eachresulting in a slightly greater enlargement of the diameter of the coremember 20 in the portion illustrated. In the case of a two passoperation an insert may be used along the wall 36 of the die 30 so as tolimit the diametrical expansion of the core member 20 to about half thefinal expansion desired.

As illustrated in Fig. 2, the core member 20 assumes the bulbousconfiguration illustrated so that its normal diameter, as for example at10, is gradually increased at l2 so as to produce an enlarged portion14, which portion is again gradually diminished in diameter through thesection i6 back again to a normal diameter at 18. The particularconfigurations of the enlarged and normal sections of the core member 20are better shown in the cross-sectional illustrations of Figs. 3 and 4.A plurality of slots 8U may be proprovided on the upper surface of thelower half of the die 3U to provide guides for pins which may be used inconjunction with templates in gauging the contour of the bulbed portionof the core member 20.

Although it may be desirable initially to begin fabrication on a hollowtubular core member which has a varying or tapering wall thickness, itis preferred that the tapering operation be performed after the bulbingprocess has been completed. To this end then, and as illustrated inFigs. 5 and 6, the enlarged section 14 of the core member 20 may haveits outer surface machined after bulbing to achieve the desired taper.

In order to obtain a desirable taper in the outboard section '18, whichsection has been maintained at the normal diameter of the original tubeduring the bulbing operation, the section 18 is subsequently coldrolled. In addition to providing a desired taper in the wall thicknessof the core section 18, the cold rolling operation serves to extend thelength of the core member back to a desirable length since, as mentionedabove, the tube is somewhat shortened during the initial bulgingoperation.

The particular expanding method and tapering method described mayreadily be performed by means of swaging and the like.

Figs. '7 to 10 illustrate the plan form configuration of the core member20 after it has been partially attened so as to provide upper and lowermajor surfaces H0 and H2. During the flattening operation dies are usedin order to get a desirable contour of the surfaces H0 and H2 that whenthe outer shell is positioned thereon the airfoil shape of the shellwill conform to, and intimately contact the major surfaces |||l and iiiof the core member 20. This flattening process may be done as a separateoperation on the core alone or simultaneously with the outer shell.

As illustrated in Figs. l1 to 15, the root end of the core member may bemachined so as to provide a plurality of bearing races |20 which willprovide cooperating blade retention mechanism for mounting the blade ina hub.

The outer shell |24 is preferably performed by doubling over a sheet ofmetal and seam welding together the major edges of the sheet to form ablade trailing edge |26. The tip or open end of both the shell and thecore member 20 may also be crimped together and sea-m welded so as toform a core tip |39 and a blade tip |32 as seen in Fig. l5. Since theouter shell I 24 is usually preformed, it is telescoped over the coremember 29 and positioned relative thereto, as for example by X-raymechanism or other means. Since the major fiat surfaces ||0 and ||2 ofthe core member 29 have been preformed to be in substantially completelyjuxtaposed relation with the shell, it is thereafter possible tointerpose solder between these mating surfaces and to gradually heat theentire assembly in a mold with a proper iiux added so that the entireassembly will be fastened together iirmly and heat treatedsimultaneously.

In order to provide for a smooth path of load transfer at the inboardend of the shell, a tab 159 (Figs. 11 and 12) may be provided so that anabrupt termination of the shell will not cause local detrimentalstresses which may tend to produce cracks in the shell in this vicinity.The edges of the tab |50 may further be smoothly bevelled as indicatedat |52 (Fig. 12) to provide a gradual diminishing of thickness of theshell material.

The particular soldering and bonding process for attaching the shell tothe core does not form a specic part of this invention and is moreclearly described and claimed in patent application Serial No. 484,254,iiled April 23, 1943, now Patent No. 2,511,858, by Glen T. Lampton.

Referring to Fig. 11, it is readily apparent that the bonding areabetween the shell |24 and the core 20 is increased adjacent the root endof the blade while at the same time a reduction has been provided in theshell overhang in a chordwise direction beyond the fore and ait edges ofthe core member 20. Further, by gradually decreasing this bonded orcontacting area (in an outboard direction) as clearly illustrated by thediverging contour of the core member 20 at point i6, stressconcentrations are maintained within acceptable limits at the outboardthreshold of the enlarged portion '14.

Also, the enlargement 74 of the core member 20 provides increasedstiiness both to the core and to the shell in the inboard semi-span ofthe blade which inboard region is obviously subjected to the greatestbending loads both in the plane of the blade and in planes transverselythereto.

Although only one embodiment of this inv-ention has been illustrated anddescribed herein, it is apparent that Various changes and modificationsmay be made in the arrangement and construction of the various partswithout depart ing from the scope of this novel concept.

What it is desired to obtain by Letters Pat ent is:

1. 1n a hollow metal propeller blade having a substantially tubularmetal core member forming the load carrying portion of said blade, meansfor mounting said blade comprising mounting elements formed integrallywith the inboard end of said core, said core comprising substantiallyflattened surfaces located outboard of said mounting elements andextending substantially to the tip thereof, a shell of airfoil shapesurrounding said core and comprising a continuous sheet surrounding saidcore and having its major edges connected together to form the trailingedge of said blade, means for bonding said shell to said core along saidflattened surfaces, and means for reducing the deiiection tendencies ofsaid blade and increasing the bonded area between said core and shellcomprising a bulbed portion of said core adjacent the inboard end of theblade and intermediate the ends thereof, said bulbed portion having across-sectional conguration whereby the circumference of said tubularcore member is gradually increased relative to the chordwise dimensionof the shell in an outboard direction from said mounting elements andsubsequently decreased a substantially like amount at a distanceoutboard along the span of said blade.

2. A propeller blade according to claim 1 wherein the outboard edges ofsaid shell are in juxtaposed relation along a continuous chordwise lineand bonded together to form the tip of said blade.

CHARLES M. KEARNS, JR. GLEN T. LAMPTON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,354,550 Jamison Apr. 19, 1932 1,869,478 Heath Aug. 2, 19321,927,247 Squires Sept. 19, 1933 1,950,080 Cierva Mar. 6, 1934 2,081,645Squires May 25, 1937 2,259,247 Dornier Oct. 14, 1941 2,262,163 BrauchlerNov. 11, 1941 2,272,439 Stanley et al Feb. 10, 1942 2,364,635 HaslerDec. 12, 1944 2,366,164 Weick et al Jan. 2, 1945 2,451,099 La Motte Oct.12, 1948 2,465,007 Bragdon et al Mar. 22, 1949 2,496,169 Lochman Jan.31, 1950 2,511,858 Lampton June 20, 1950 2,511,862 Martin June 20, 1950FOREIGN PATENTS Number Country Date 877,664 France Sept. 14, 1942

